Conventional systems for controlling the mass flow rate of fine materials, such as particulate coal, supplied as a fuel into for example a furnace, such as a coal fired boiler, generally employ load cell based weight rate measuring systems to control the gross mass flow rate of the coal to boilers such as the system described in U.S. Pat. No. 4,490,077. However, such systems do not disclose rapid manual or automatic on-line recalibration of mass flow rate controlling systems within fractions of a second to accommodate changing operating conditions as required by the present invention.
U.S. Pat. No. 3,635,082 describes the determination of mass flow from velocity and density measurements of a coal and gas stream using two capacitance transducers mounted a known distance apart in the supply line between a coal storage vessel and furnace. The measurement system described by this patent relies on large signal variations, including introducing a slug of compressed gas into the supply line to produce a marker gap, to observe "slugs" of coal in a gas stream in an industrial process. Such a system would not be compatible with the present invention which requires a uniform mass flow rate of coal introduced to the gasifier over periods of time of approximately 5 seconds or less.
Other conventional systems control the mass flow rate by determining the mass flow rate indirectly via optical measurement of the coal concentration within a conduit leading to the furnace. Optical measurements, such as radiation absorption for light, are limited to applications of low coal density suspensions, say less than 10 kg/cubic meter, since light must be transmitted through the coal mixture present in the conduit. For example, German patent publication No. 2554565 describes an indirect method of deriving mass velocity of coal transported to a gasifier by optically measuring the coal concentration at a point in the transport line and from the velocity of separately supplied carrier gas. First, the coal entrains gas as it passes from a storage vessel into a transport line. Additionally, the carrier gas, which is not measured, is introduced to assist the coal in discharging from the vessel to the transport line en route to the gasifier. Since it is the total gas stream in the transport line in addition to other factors which govern the mass flow rate, the invention described by this patent could not be used to control the mass flow rate of coal to a gasifier within the desired accuracy, say plus or minus 2 percent, operated with varying suspension densities of 50-800 kg/cubic meter, which are characteristic of various coal types and at various flow conditions.
The present invention is directed to overcoming this problem in the prior art.
Applicants are not aware of any prior art which, in their judgment as persons skilled in this particular art, would anticipate or render obvious the present invention. However, for the purpose of fully developing the background of this invention, and establishing the state of requisite art, the following art is set forth: U.S. Pat. Nos. 3,689,045; 4,582,454; 4,488,837; Re 31,676; 4,004,647; 4,482,275, and Chemie-Ing. Tech. 47 105 (1975).